It has become increasingly common in the silicon integrated circuit (IC) industry to design so-called System on Chip (SoC) solutions. One driving force behind this trend is the ever present desire to minimise the chip area required to perform the required functionality of the chip, and hence ultimately reduce the size and weight of the electronic devices into which the chip is incorporated.
Reducing the size of the components on the chip is not the only problem encountered in realising a more compact design. Another problem is that the components must be positioned closer to each other on the chip. This is problematic for circuitry surrounding inductors, because inductors radiate magnetic fields that interfere with the operation of the surrounding circuitry. This interference issue is particularly problematic when two or more inductors are located on the same chip. In this situation, each inductor couples with the other inductors (called cross coupling). This is as a result of the current flowing through each inductor inducing a magnetic field which radiates to the other inductors and induces voltages (and so currents) in those inductors. This cross coupling changes the operation of all the inductors involved. For example, if an inductor is used in a voltage controlled oscillator (VCO) then cross coupling from other inductors changes the current through the inductor and hence changes the resonant frequency of the VCO.
It is not possible to prevent a magnetic field from being created by an on-chip inductor. However, efforts have been made to alleviate the cross-coupling issue when designing the layout of a chip comprising more than one inductor. One approach taken is to locate inductors on a chip as far apart from each other as possible given other design constraints. Since the strength of the magnetic field radiated from an inductor is inversely proportional to the cube of the radial distance from the inductor, the greater the separation of inductors on a chip, the less they interfere with each other's operation.
Separating the inductors on a chip only results in a partial reduction of their cross-coupling. Additionally, inductors are often adjacent components in a signal path. Thus, separating these inductors on the chip requires additional wiring to connect them together. If, due to other design constraints, inductors have to be physically close to each other on a chip, then typically other circuitry is incorporated on the chip in order to counter the effects of the cross-coupling. For example the output of a VCO may be frequency shifted outside the VCO with additional circuitry in order to counter the pulling effect of a nearby inductor. Such additional circuitry drains power and utilises chip area. This solution is not desirable given market demand for ever smaller and lower power/longer battery life electronic devices. Additionally, as chips get smaller to satisfy market demand, an inevitable consequence is that the maximum separation of inductors decreases, and hence the cross-coupling effects worsen.
Thus, there is a need for an improved inductor configuration on an integrated circuit chip which alleviates cross-coupling using a lower power, more space-saving solution than those described above.